To control microbial growth on a surface, antimicrobials are applied to the surface. Microbial growth in a material can also be controlled by mixing antimicrobials with the material.
One group of compounds commonly used for surface disinfection are quaternary ammonium compounds ("QACs"). The general formula for the QACs is as follows: ##STR1## where R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are alkyl groups which may be alike or different. Structurally, these compounds contain four carbon atoms linked directly to one nitrogen atom through covalent bonds. The portion attached to the nitrogen by an electrovalent bond may be any anion, but it is usually chloride or bromide to form the salt. The nitrogen atom with the attached alkyl groups forms the positively charged cation portion. Depending on the nature of the R groups, the anion and the number of quaternary nitrogen atoms present, the antimicrobial quaternary ammonium compounds may be classified as monoalkyltrimethyl, monoalkyldimethylbenzyl, heteroaromatic, polysubstituted quaternary, bis-quaternary, or polymeric quaternary ammonium compounds.
Quaternary ammonium compounds have been widely used for disinfection of floors, walls, and equipment surfaces in hospitals, nursing homes, and other public places. Disinfection of these surfaces is carried out by various means, such as an aerosol spray containing quaternary ammonium compounds or a mist of quaternary ammonium compound germicide generated from commercial fogging devices. In addition, QACs have been used in the treatment of food contact surfaces and in outdoor swimming pools and cooling water systems to prevent the proliferation of bacteria. John J. Merianos, "Quaternary Ammonium Antimicrobial Compounds", in Disinfection, Sterilization and Prevention, 4th ed. Lea & Fabiger, Philadelphia (1991), which is hereby incorporated by reference.
When QACs are applied directly to surfaces, their effect is not long-lasting due to leaching of the compound from the surface. Therefore, frequent applications may be needed to achieve prolonged antimicrobial effects. As a result, the use of QACs as antimicrobials has not proven to be suitable for surfaces that are not accessible for repeated applications.
The ability to modify the surface of smectite clay minerals by means of a cation exchange with organic cations has been recognized since the 1930s (Gieseking, J. E., "Mechanism of Cation Exchange in the Mont-Morillonite-Beidellite-Nontronite Type of Clay Minerals," Soil Science, 47:1-14 (1939), which is hereby incorporated by reference). The quantity of exchangeable cations available on the clay mineral surfaces is given by the cation exchange capacity ("C.E.C."), which has units of milliequivalents (meq)/100 grams of clay. The C.E.C. of smectite clays typically varies between approximately 50 and 150 meq/100 grams. The cation exchange is accomplished by dispersing the clay mineral in water or a mixture of water and a miscible organic solvent such as alcohol in which is dissolved a quantity of the organic cation sufficient to satisfy the C.E.C. of the clay mineral. The organic cation replaces the inorganic cation in a nearly quantitative manner by this single treatment method.
Occasionally, clay minerals are treated with solutions containing an organic cation at greater than the C.E.C. value (as disclosed in U.S. Pat. Nos. 4,116,866, 4,081,496, 4,105,578, and 4,287,086 to Finlayson; U.S. Pat. Nos. 4,365,030 and 4,317,737 to Oswald, et al.; and U.S. Pat. No. 4,929,644 to Guilbeaux; which are hereby incorporated by reference). However, treatment with an organic cation in excess of the C.E.C. does not ensure that the product will have the exchanged organic material affixed to the clay surfaces in excess of the C.E.C. To definitively establish that the attached organic material exceeds the C.E.C., it is necessary to analyze the quantity of organic material in the organo-clay product. This can be done by determining the carbon content of the product (Ohashi, et al., "Antimicrobial and Antifungal Agents Derived From Clay Minerals: Part IV Properties of Montmorillonite Supported by Silver Chelate of Hypoxanthine," J. Mat. Sci., 27:5027-30 (1992) and Ohashi, et al., "Antimicrobial and Antifungal Agents Derived From Clay Minerals: Part VIII Thermostability of Montmorillonite Intercalated by Silver Chelate of 2-(4-thiazolyl)-benzimidazole or Quaternary Ammonium Salts," J. Mat. Sci., 31:3403-07 (1996), which are hereby incorporated by reference) or by determining the weight loss upon high temperature oxidation of the product.
The typical procedure used to form the organo-clay is to treat the clay mineral by a single exposure to an aqueous solution of the organic cation whose total quantity in the solution is used to determine the C.E.C. equivalents bound to clay mineral (U.S. Pat. No. 4,365,030 to Oswald, et al., which is hereby incorporated by reference).
The utility of organo-clay as antimicrobial compounds has been pointed out by several workers (Oya, et al., "An Antimicrobial and Antifungal Agent Derived From Montmorillonite," Appl. Clay Sci., 6:135-42 (1991); Oya, et al., "Antimicrobial and Antifungal Agents Derived From Clay Minerals (III): Control of Antimicrobial and Antifungal Activities of Ag+-exchanged Montmorillonite by Intercalation of Polyacrylonitrile," Appl. Clay Sci., 6:311-18 (1992); Ohashi, et al., "Antimicrobial and Antifungal Agents Derived From Clay Minerals (II): Properties of Montmorillonite Supported by Silver Chelates of 1,10-phenanthroline and 2,2'-dipyridyl," Appl. Clay Sci., 6:301-10 (1992); and Ohashi, et al., "Antimicrobial and Antifungal Agents Derived From Clay Minerals: Part VIII Thermostability of Montmorillonite Intercalated by Silver Chelate of 2-(4-thiazolyl)-bebzunudazole or Quaternary Ammonium Salts," J. Mat. Sci., 31:3403-07 (1996), which are hereby incorporated by reference). These materials were fabricated by treating the smectite clay mineral with an organic cation or a metal (silver) chelated with an organic cation, all at a quantity sufficient to satisfy the C.E.C. of the clay mineral.
However, none of these references disclose a clay mineral having an organic cation present in excess of the C.E.C. of the clay. Accordingly, the clay materials of the prior art are not entirely satisfactory as compounds having antimicrobial activity.
The present invention is directed to overcoming these above-noted deficiencies by disclosing a process which attaches an antimicrobial ligand to the clay in excess of the C.E.C., thereby imparting increased antimicrobial activity.